1. Field of the Invention
The present invention relates to a novel ethylene-aromatic vinyl compound copolymer and a method for its production.
2. Discussion of Background
For the production of a copolymer of ethylene with an aromatic vinyl compound such as styrene, studies have been conducted primarily by using so-called heterogeneous Ziegler-Natta catalysts (e.g. Polymer Bulletin 20, 237-241 (1988)). However, conventional heterogeneous Ziegler-Natta catalyst systems are not so practical, since the catalytic activities are low, the styrene content in the product is low, or the product does not have a uniform regular copolymer structure or contains a substantial amount of homopolymers.
Further, some ethylene-styrene copolymers obtainable by using so-called homogeneous Ziegler-Natta catalyst systems comprising a catalyst of a transition metal compound and an organoaluminum compound, and methods for their production have been known.
JP-A-3-163088 and JP-A-7-53618 disclose styrene-ethylene copolymers wherein no normal styrene chain is present i.e. so-called pseudo random copolymers, obtained by using a complex having a so-called constrained geometrical structure. Here, a normal styrene chain is meant for a head-to-tail bond chain.
However, phenyl groups in the alternating structure of ethylene-styrene present in such pseudo random copolymers, have no stereoregularity.
Hereinafter, styrene and cyclopentadienyl may sometimes be represented by St and Cp, respectively.
JP-A-6-49132 and Polymer Preprints, Japan 42, 2292 (1993) disclose methods for producing similar styrene-ethylene copolymers wherein no normal St chain is present, i.e. so-called pseudo random copolymers, by using a catalyst comprising a bridged Cp type Zr complex and a cocatalyst.
However, according to Polymer Preprints, Japan 42, 2292 (1993), phenyl groups in the alternating structure of ethylene-styrene present in such pseudo random copolymers, have no substantial stereoregularity.
On the other hand, a styrene-ethylene alternating copolymer obtainable by using a Ti complex having a substituted phenol type ligand, is known (JP-A-3-250007 and Stud. Surf. Sci. Catal. 517 (1990)). This copolymer has a feature that it consists essentially of an alternating structure of ethylene-styrene and contains substantially no other structure such as an ethylene chain, styrene chain, a structure comprising an ethylene chain and styrene, a structure of e.g. a head to head or tail to tail bond of styrene. The alternating degree (value .lambda. in the present specification) of the copolymer is at least 70, substantially at least 90.
Namely, the resulting copolymer is a copolymer having a very high degree of alternation and consisting substantially solely of the alternating structure, whereby it is substantially difficult to change the compositional ratio of the copolymer consisting of 50% of ethylene and 50% of styrene by molar fraction.
Further, the stereoregularity of phenyl groups of which copolymer is isotactic, but the isotactic diad index m is about 0.92.
Further, the weight average molecular weight of this copolymer is low at a level of 20,000, which is inadequate to provide practically useful physical properties. It should also be added that the catalytic activities are very low, and the copolymer can hardly be regarded as practically useful, since it is obtained as a mixture with e.g. syndiotactic polystyrene.
Further, Macromol. Chem., 191, 2387 (1990) has reported a styrene-ethylene copolymer prepared by using CpTiCl.sub.3 as a transition metal compound and methyl alumoxane as a cocatalyst. It is disclosed that at a certain specific transition metal compound/cocatalyst ratio, a pseudo random copolymer free from a styrene chain can be obtained although the catalytic activities are very low, but no disclose is given with respect to the stereoregularity of the ethylene-styrene alternating structure of the resulting copolymer.
In Eur. Polym. J., 31, 79 (1995), copolymerization of ethylene and styrene is carried out under various conditions using the same catalyst. However, it is disclosed that obtained are syndiotactic polystyrene and polyethylene only, and no copolymer is obtainable.
Macromolecules, 29, 1158 (1996) discloses that copolymerization of ethylene and styrene is carried out by means of CpTiCl.sub.3 and a boron type cocatalyst to obtain a copolymer having a high degree of alternation together with syndiotactic polystyrene and polyethylene. However, no stereoregularity is observed in the ethylene-styrene alternating structure.